Is China's currency policy like the age of a child, who after a birthday can never go back? Or is it more like a reservoir that rises during the rainy season, and falls during a drought?

It's like a reservoir, of course. At least, that's according to the country's central bank, speaking in an official statement after being accused by President Donald Trump's administration of currency manipulation.   

The point that Chinese central-bank officials were trying to make was that the country's exchange rate, while carefully managed by authorities as part of historically state-directed economic policies, is nevertheless subject to market forces -- say, for example, from investors moving money out of the country based on forecasts for slowing growth.

Therefore, the logic goes, the yuan's depreciation below 7 per U.S. dollar earlier this week wasn't an act of manipulation, but an outcome of natural fluctuations in global currency markets. 

"It should be noted that the exchange rate, though now over 7, is not like our age that only grows and never falls, nor is it like a dam which allows water to rush down torrentially once broken through," according to the official statement. "It is more like a reservoir level rising in wet season and falling in dry season, which is nothing but normal."

The use of such analogies and proverbs in an official central-bank pronouncement might elicit snickers from investors in Western countries, including those now scrutinizing every twist and turn in Trump's trade war with China for clues on future market indicators from Federal Reserve interest rates to the S&P 500.  

But the country's usage of such elaborate language may speak to a simpler truth as Trump ratchets up his rhetoric and threats of additional tariffs: China isn't backing down.

According to a statement Monday from the U.S. Treasury Department, China has a "long history of facilitating an undervalued currency through protracted, large-scale intervention in the foreign exchange market."

But in recent days, according to the statement, China has "taken concrete steps to devalue its currency, while maintaining substantial foreign exchange reserves despite active use of of such tools in the past."

In other words, the fact that China actively managed its currency in the past, and now is refraining from such active management, could itself be considered a form of manipulation. 

Given Trump's pattern thus far of gradually escalating tensions with China, that could mean that the ongoing trade war between the world's two biggest economies could last for a while.

"China has consistently refused to buckle under pressure," Ethan Harris, global economist for Bank of America, the second-biggest U.S. bank, wrote in a recent report. "Why should this time be different?

As a Chinese proverb goes, however, once you pour the water out of the bucket, it's hard to get it back in.